1. Field of the Invention
The present invention generally relates to frictionally engaged assemblies of parts which are subject to intermittent accelerations and, more particularly, to arrangements for the clamping together of parts which are subject to degradation due to application of high levels of internal stress over time in which the forces caused by the clamping arrangements are maintained at minimal levels even during acceleration.
2. Description of the Prior Art
Many devices are known in which some of the constituent parts are held together in precise juxtaposition by clamping arrangements of various forms. Clamping arrangements and frictional engagement between parts are often preferred since precise alignment in the mounting plane can be more readily achieved. However, facilitating precise alignment in such a fashion with clamping arrangements often makes the assembly subject to relative shifting of parts in the mounting plane under strong acceleration forces having vector components in the mounting plane (hereinafter referred to as “in-plane” forces) such as impacts on the assembly or its supports (e.g. packaging) either in use or during shipping. This tendency, while undesirable, is often tolerated where one or more of the clamped parts is made of a more or less brittle material and repairable loss of alignment is preferable to breakage of a part during unexpected accelerations while the parts may be maintained in accurate alignment under anticipated in-plane accelerations. For example, a shipping specification may be to withstand accelerations of either three or six “G's” (e.g. three or six times the force of gravity on the mass of the part) during shipping which may be accommodated by the device/assembly, its mounting or protective packaging or a combination thereof. Many clamping arrangements are known which may be used singly or in combination to develop static forces to counteract or withstand virtually any anticipated acceleration up to the level of producing damage to individual parts without relative shifting of parts while providing a degree of protection from breakage.
For an optical system relying on a clamping system to secure optics in place and in alignment, large clamping forces may be required to meet the 6 G specification where the clamping force must at least equal the mass of the optic multiplied by six times the acceleration of gravity. Due to various factors such as available space, materials costs and the like, space for holding the optics may be very small. Large clamping forces applied over small areas for holding or mounting the optic also generate high levels of stress. Unfortunately, some materials from which some parts of an assembly may be made may be subject to degradation of important properties when subjected to stresses for substantial periods of time.
Perhaps more importantly for some optical systems, glass will exhibit stress-induced birefringence (which can be observed, for example, as colors or color fringes when the object is placed between crossed polarizers and transilluminated) which will gradually become permanent over time. Permanence of stress-induced birefringence can also be caused by a relatively few limited temperature excursions in a potentially short period of time or a single temperature excursion if sufficiently great while stress is applied. This phenomenon is well-recognized and has been used for the study of stresses in solid objects for many years. However, in systems where permanent stress induced birefringence is not a desired result and latent and constant stresses are present, it should be recognized that small and uncontrolled changes in environmental conditions can unpredictably accelerate the occurrence of permanent stress-induced birefringence and render an optical system, for example, unusable in a very short time.
Some materials used for the optics may be vulnerable to high stress under accelerations and may fail or break under those conditions. Varying the clamping force, and ultimately the stress, during accelerations may reduce the possibility of damage to the optic.
In other applications such as for clamping of a wafer and/or reticle (both of which are generally subjected to repeated movement and accelerations when in use) in a projection lithography tool where the clamped part has a crystalline structure, small imperfections in the crystal lattice may be caused and/or propagated leading to increased brittleness and possibly breakage.
The amount of force which must be placed on an object to maintain it in a given relative position under a given maximum specified or anticipated acceleration can vary greatly, depending on the material and elasticity of contiguous parts and the mass of the clamped part. Therefore, the amount of stress which is induced in a part by such a force also may vary greatly but once determined and developed by springs, piezoelectric devices and the like which are known for clamping the parts together, generally remains unchanged notwithstanding the fact that important properties of the part(s) may be degraded over time by such a force. In any case, the force required to prevent relative shift of parts under acceleration is greater than the force required to do so when the assembly is at rest, but, by the same token, minimization of clamping forces to limit degradation over time will be insufficient to maintain alignment of parts when an acceleration is encountered.